This invention relates to FM stereophonic broadcasting systems and, more particularly, to adaptive expanders for FM stereophonic broadcasting systems utilizing companding of the difference signal.
It is known from U.S. Pat. No. 4,485,483 of Emil L. Torick and Thomas B. Keller, the disclosure of which is hereby incorporated herein by reference, to compand the difference signal in a modulated channel that is in quadrature with the channel normally used for stereo in such a way as not to increase the bandwidth requirements for transmission. In the system disclosed in this patent (hereinafter sometimes referred to as the "Torick/Keller system") the usual left and right signals are conventionally matrixed to obtain conventional sum (M) and difference (S) signals. The difference signal is used to amplitude-modulate a first sub-carrier signal and at the same time is applied to a compressor which compresses its dynamic range in accordance with a given law to produce a compressed difference signal S'. The compressed difference signal S' is used to amplitude-modulate a second sub-carrier signal, preferably of the same frequency but in quadrature phase relationship with the first. Suppressed-carrier, double-sideband modulation of each sub-carrier is employed, with the frequency of the sub-carrier signal being sufficiently high to assure a frequency gap between the lower sidebands of the modulated sub-carrier signals and the M signal. A conventional low-level phase reference pilot signal of a frequency lying within the frequency gap is employed for detection purposes at the receiver. The M signal, the two modulated sub-carrier signals, and the pilot are modulated onto a high-frequency carrier for transmission purposes. The receiver includes a demodulator for deriving the M signal, the normal difference signal S and the compressed difference signal S', and an expander for complementarily expanding the derived compressed difference signal. The expanded noise-reduced version of the difference signal is combined with the derived sum signal M to obtain the original left (L) and right (R) signals. The receiver also includes switch means for applying the normal difference signal S, instead of the expanded version of the derived difference signal, to the combining means for enabling the receiver for reproduction of conventional stereophonic signals. Companding of the difference signal S gives 22 dB to 26 dB signal-to-noise improvement in the transmission and enables the stereo listener to enjoy the same signal-to-noise ratio as does the conventional monophonic listener. This amount of reduction of received noise greatly increases the effective stereo service area by improving the quality of the signal received by listeners located within the service area.
Commonly assigned U.S. patent application Ser. No. 688,940 filed concurrently herewith by applicant David W. Stebbings, the disclosure of which is hereby incorporated herein by reference, discloses an improved FM stereo system that is similar to the Torick/Keller system in that the usual difference signal S and a compressed difference signal S' are both transmitted. However, unlike the Torick/Keller system, in which only the expanded version of the received compressed difference signal is matrixed with the sum signal M to obtain the original L and R signals, the receiver according to the Stebbings disclosure matrixes a noise-reduced difference signal, derived by combining and expanding the received difference signals S and S', with the derived sum signal M to obtain the original L and R signals. Since the signal content of the signals S and S' is the same, by combining them the effective level of the received difference signal is increased by 6 dB, whereas the noise is increased by only 3 dB, resulting in a net 3 db improvement in signal-to-noise ratio.
The transmission of the uncompressed difference signal S (necessary for compatibility with existing systems), coupled with the utilization at the receiver of both the uncompressed and the compressed difference signals, allows any desired companding law to be used in the compressor for the difference signal. For example, instead of being limited to the 2-to-1 slope typical of the "CX" companding system described in commonly assigned U.S. Pat. No. 4,376,916, or to the characteristics of other known companding systems, it is possible to use a compressor having an infinity-to-one or similar companding characteristic which provides a subjectively assessed 10 dB to 12 dB improvement against program modulated noise for a given amount of noise reduction over that realizable with such prior art companders.
A primary object of the present invention is to provide an improved FM stereophonic broadcasting system which better utilizes the greater channel capacity of the Torick/Keller system while still realizing its improved signal-to-noise advantage.
Another object of the invention is to provide an adaptive expander for use in the receiver having the capability of adapting to any compressor characteristic that may be employed at the transmitter.
Yet another object is to provide an expander for use in an FM stereophonic system that is capable of adaptive frequency response decoding and adaptive attack and recovery time decoding so that all parameters of the original signal can be restored automatically regardless of the compression law used in transmission.